This document offers two methods to scan for radar signals across
802.11a outdoor channels before deployment of mesh networks. One based on
4.0.217.200 image, the other using newer functionality on the mesh released, in
particular 4.1.192.17M. It covers both 1520 and 1510 mesh access point
families.

The objective is to provide a mechanism to check for possible radar
signals that can affect a wireless mesh network that uses 802.11a as backhaul
links.

It is important to validate the presence of radar on any wireless mesh
deployment. If during operation, an access point (AP) detects a radar event
over the Radio Frequency (RF) channel that the network backhaul uses, it must
immediately change to another available RF channel. This is dictated by Federal
Communications Commission (FCC) and European Telecommunications Standards
Institute (ETSI) standards, and is established to allow the sharing of the 5
GHz spectrum between wireless LAN (WLAN) and military or weather radars that
use the same frequencies.

The effects of radar signal over a wireless mesh network with 802.11a
backhaul can be different. This depends on where the radar is detected and on
the state of “full sector DFS mode” configuration setting (in
case it is disabled):

If a mesh access point (MAP) sees the radar on the current channel,
it goes silent for one minute [dynamic frequency selection (DFS) timer]. Then,
the MAP starts to scan channels for a suitable new parent to associate again to
the mesh network. The previous channel is marked as not usable for 30 minutes.
If the parent [other MAP or rooftop access point (RAP)] does not detect the
radar, it remains on the channel and is not visible for the MAP that did detect
it. This situation can occur if the detecting MAP is closer or in line of sight
of the radar, and the other APs are not. If no other parent is available in
another channel (no redundancy), the MAP remains off network for the 30 minutes
of the DFS timer.

If a RAP sees the radar event, it goes silent for one minute, and
then selects a new channel from the 802.11a Auto RF channel list (if currently
joined to controller). This causes this section of the mesh network to go down,
as RAP has to change channel, and all the MAPs have to search for new parent
location.

In case that full sector DFS is enabled:

If a MAP sees the radar on the current channel, it notifies RAP of
the radar detection. The RAP then triggers a full sector channel change (RAP
plus all its dependant MAPs). All devices after going into the new channel, go
silent for one minute, to detect for possible radio signals on the new channel.
After this time, they resume normal operation.

If a RAP sees the radar event, it notifies all MAPs for a channel
change. All devices after going into the new channel, go silent for one minute,
to detect for possible radio signals on the new channel. After this time, they
resume normal operation.

The feature of “full sector DFS mode” is available on mesh releases
4.0.217.200 and later. The main impact is that the full sector will go one
minute on silent mode after channel change (mandated by DFS), but it has the
advantages that it prevents MAPs to become isolated if they detect radar, but
its parent not.

It is advisable that before you plan and install, contact the local
authorities in order to obtain information if there is any known radar
installation nearby, such as weather, military, or an airport. Also, in
harbors, it is possible that passing or incoming ships might have radar that
affects the mesh network, which might not be present during the survey phase.

In case that severe radar interference is detected, it is still
possible to build the network using 1505 APs. This is instead of using 802.11a
radio as backhaul. The 1505 APs can use 802.11g, sharing it with the client
access. This represents a technical alternative for sites too close to a
powerful radar source.

On most situations, removing the affected channels can suffice to have
a operable network. The total number of channels affected depends on radar
type, and distance from the deployment site to the radar source, line of sight,
etc.

Note: If the method proposed in this document is used, it does not make any
warranties that there is not radar in the tested area. It constitutes a initial
test to prevent possible issues after deployment. Due to the normal variations
on RF conditions for any outdoor deployment, it is possible that the detection
probability can change.

The information in this document was created from the devices in a
specific lab environment. All of the devices used in this document started with
a cleared (default) configuration. If your network is live, make sure that you
understand the potential impact of any command.

It is important to think of the radar energy as a light source.
Anything that can be on the path to the survey tool, from the radar source, can
generate a shadow or completely hide the radar energy. Buildings, trees, etc
can cause signal attenuation.

Doing the capture indoors is not a substitution for a proper outdoor
survey. For example, a glass window can produce 15 dBm of attenuation to a
radar source.

No matter what kind of detection is used, it is important to select a
location that has the least obstructions around, preferably near where the
final APs will be located, and if possible at the same
height.

As the 1500 AP is added to the controller, you should know the MAC
address, so it can be authorized. The information can be gathered from the
sticker on the AP, or by using the debug lwapp errors
enable command on the controller in case the AP is already
installed. As the AP is not yet authorized, it is possible to easily see the
MAC address:

Disable the radio of the AP with the config 802.11a
disable <APNAME>
command.

Select a channel, then manually set the 802.11a radio on it. Cisco
recommends starting from the highest channel (140), and then decreasing toward
100. Weather radar tends to be on higher channel area. Use the
config 802.11a channel <APNAME><CHANNELNUM>
command.

Enable the 802.11a radio of the AP with the config
802.11a enable <APNAME>
command.

Wait until the radar debug is generated, or a “safe” time, for
example 30 minutes in order to make sure there is no fixed radar on that
channel.

Disable the radio of the AP with the config 802.11a
disable <APNAME>
command.

Select a channel, then manually set the 802.11a radio on it. Cisco
recommends to start from the highest channel (140), then decrease toward 100.
Weather radar tends to be on higher channel area. Use the config
802.11a channel <APNAME><CHANNELNUM>command.

Enable the 802.11a radio of the AP with the config
802.11a enable <APNAME>
command.

Wait until the radar trap is generated, or a “safe” time, for
example 30 minutes in order to make sure there is no radar on that
channel.

The AP keeps the information on what channels have seen radar for
30 minutes, as required by regulation. This information can be seen from the
GUI interface on the controller in Monitor > 802.11a Radios
page.

Select the AP used for channel testing and scroll down to the
bottom of the frame:

For additional details on the radar signals found by the WLC
debug commands described earlier, use the Cognio
Spectrum Analyzer in order to validate. Due to the signal characteristics, the
software does not generate an alert on the signal itself. However, if you use
the Real Time FTT “max hold” trace, you can obtain a picture and verify the
number of detected channels.

It is important to take into consideration that the antenna gain, the
sensitivity of the 1510 AP's 802.11a radio, and the Cognio sensor are
different. Therefore, it is possible that the reported signal levels differ
between what the Cognio tool and the 1510 AP report.

If the radar signal level is too low, it is possible that it is not
detected by the Cognio sensor because of lower antenna gain.

Make sure that no other 802.11a devices are active that can affect the
capture; for example, the Wi-Fi card in the laptop used during the test.

In order to perform the capture, go to the Cognio Spectrum Expert, and
set these parameters:

Use the external antenna.

In Tools, go to Settings. Choose Band and Channel
Settings, then select your regulatory domain, and only check the
802.11a box. Then, click
OK.

Click the Real Time FFT plot in order to select
it.

In the Control Panel, verify that the Trace 3 is
On, and set to Max Hold.

In the same section, verify that the Frequency is set to
Center Freq/Span, and the band is 5.47 – 5.726 Ghz
Band.

After enough capture time, the max hold trace shows the radar
signal characteristics:

Use the start/stop settings available in the Control Panel in order
to zoom into the signal plot.

This allows you to get more details on the total channels affected
and power of the signal:

It is possible to customize the default 802.11a channel list.
Therefore, when a RAP is connected to the controller, and it is necessary to do
a dynamic channel selection, the previously known affected channels are not
used.

In order to implement this, it is only necessary to change the Auto RF
channel selection list, which is a global parameter to the controller. The
command to use is config advanced 802.11a channel delete
<CHANNELNUM>. For example: